London terror attack: Latest pictures

4 dead and 40 injured in London terror attack

Armoured police personnel carriers are seen on a street leading to the Houses of Parliament in central London on March 24, 2017 two days after the March 22 terror attack on the British parliament and Westminster Bridge

They can also send an alert to their Facebook contacts in order to check that they are OK.

First launched in 2014, Safety Check was started by Facebook to help the victims of natural disasters, and was initially used to help the recovery of victims from earthquakes in Japan, Chile and Nepal.

Today's action marks the first time that the feature has been used in the UK.

FACEBOOK

Facebook Safety Check lets you mark yourself as safe

Facebook also turned on Safety Check following the terrorist attacks in Paris last October in which 130 people died, allowing those caught up in the tragedy to let their loved ones know they were safe.

The company had been criticised for its apparent refusal to use Safety Check for a number of major incidents, but a Facebook spokesperson said that it could soon be able to allow individual users to turn on the feature themselves.